How Does a GC-MS Work?

Gas chromatography (GC) and mass spectrometry (MS) are two instrumental methods of scientific analysis. Used concurrently, a GC-MS instrument will separate out the individual components of a mixture in order to tell you:

What a substance is composed of

How much of each of the components is present

The Gas Chromatograph (GC)

A gas chromatograph (GC) comprises of a heated inlet port, an oven, and a fused silica column (essentially a coiled glass tube) which has been coated with a special material called the stationary phase.

Sample Preparation

Samples are generally dissolved or diluted in a solvent and then injected onto the inlet port. Other methods of sample preparation, such as solid phase extraction (SPE) and/or derivatisation may also be required.

Vapourisation

The liquid sample is vapourised in the hot inlet and becomes a gas.

Separation

The mobile phase (which is an inert gas such as helium) carries the sample through the column. Different substances in the sample interact differently with the column’s stationary phase, depending on their chemistry. This causes them to travel through the column at different speeds, thus separating them.

Detection

The separated compounds then leave the column one after the other, and enter a detector, such as a mass spectrometer (MS). The time taken for a compound to travel through the column is called its retention time.

The GC produces a graph called a chromatogram, where each separated substance is represented by a peak. The number of peaks shows the number of separated compounds in the sample. The position of each peak shows the retention time for each compound.

A mass spectrometer (MS) is commonly used as a GC detector.

The MS will break each separated compound coming from the GC into ionised fragments. To do this, a high energy beam of electrons is passed through the sample molecule to produce electrically charged particles or ions. These fragments can be large or small pieces of the original molecule. Each charged fragment will have a certain mass. The mass of the fragment divided by the charge is called the mass to charge ratio (m/z).

The fragments then go through a process of acceleration and deflection whilst traveling through a short tunnel and being exposed to a magnetic field.

They eventually hit a detection plate at the end of the tunnel, where the mass to charge ratio (m/z) and relative abundance (how much of that fragment was present in the sample) is calculated.

The MS produces a graph called a mass spectrum, which shows the signal intensity or abundance of each detected fragment’s mass to charge ratio. The mass spectrum produced by a given chemical compound is the same every time it is analysed, and thus can be considered a “fingerprint” for the molecule, allowing the compound to be identified.

Overall, a compound is identified via GC-MS not only by comparing its retention time to a standard (GC), but also by using its mass spectrum, making this an extremely powerful analytical tool.

Our Analytical Laboratories

The GC-MS is just part of our state-of-the-art laboratories. Our purpose built Analytical Laboratories and our unique approach applies multiple methods of chemical analysis and state-of-the-art scientific instrumentation to provide solutions to our client’s technical issues.

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